1. Field of the Invention
The present invention relates to a technology of detecting a periodic change in speed of a latent image carrier included in an image forming apparatus.
2. Discussion of the Background Art
In an image forming apparatus employing an electrophotographic system, such as a copier, a printer, a facsimile, a duplicator, and the like, as one of abnormal images caused by a driving and conveying mechanism, a banding phenomenon is exemplified, which causes unevenness of density in a band state periodically or at random in a sub scanning direction.
A mechanism of an occurrence of the banding phenomenon is initially described. There usually exists a vibration source in a driving system for driving a polygon mirror, a developing apparatus, and a fixing apparatus. Also driven by the driving system are a photoconductive member, a transfer member, such as a transfer conveyance belt, and an intermediate transfer belt, and the like The vibration travels from the vibration source toward the photoconductive member or the transfer member, thereby, density unevenness likely occurs at any one of exposing, developing, and transferring steps.
As a countermeasure against such problem, it is essentially most efficient to suppress vibration transmitted to the photoconductive member or the transfer member. Especially, the vibration arrived at the photoconductive member is needed most to be reduced, because it creates a change in speed and can be a cause of banding at all of the steps of the exposing, developing, and transferring.
To reduce the change in speed of the photoconductive member, it is needed to know a type of a speed change as a first thing. Conventionally, the speed change is generally known by either attaching a rotary encoder to a driving shaft of the photoconductive member or arranging marks on one end of the photoconductive member at a constant interval and detecting the marks with an optical sensor or the like. That is, the one end is used as an encoder in the latter situation. Such a system is indeed capable of wide range detecting from low to high frequencies of a speed change of the photoconductive member. However, the system is costly, because the encoder needs a high-resolution for detecting a high frequency change in speed.
The Japanese Patent Application Laid Open No. 2005-338835 discusses a method capable of detecting a high frequency (i.e., a short cycle) of a speed change with a simple sensor. Technically speaking, the method rather detects just banding and not the speed change of the photoconductive member.
Further, the Japanese Patent Application Laid Open No. 2005-17768 discusses an image forming apparatus that is capable of detecting a periodical speed change of a photoconductive drum using latent image patterns.
However, the method of the Japanese Patent Application Laid Open No. 2005-338835 needs to spend toner per detection, because an optical sensor provided needs to detect a visualized image, such as a toner image, and the like Whereas in the Japanese Patent Application Laid Open No. 2005-17768, there is no such problems of spending the toner for the purpose of detection, because they use the latent image patterns. However, since speed change during a relatively long cycle such as one rotation cycle of a drum is a detection objective, a pattern latent image including a number of lines is depicted over one cycle of the speed change and then detects a fineness of an interval of the pattern. Further, a latent image detection device needs a high resolution when detecting a speed change of a short cycle, such as high frequency banding, and the like